I 82 STEBBINS 



hybrid polyploids, which breed true to their new, intermediate condition. 

 Fertile hybrids between races or subspecies of the same species can, by 

 doubling their chromosome number, become "buffered" against too rapid 

 dissipation of their intermediate genotypes by genetic segregation, since at 

 the tetraploid level segregation is much more complex and each gene has a 

 smaller effect on variation than it does at the diploid level. 



On the other hand, the larger number of genes or alleles present in poly- 

 ploids tends to reduce the effectiveness of new gene changes or mutations, 

 and consequently progressive evolution in the direction of entirely new 

 mechanisms of adaptation is slower in polyploids than in diploids. For this 

 reason, polyploid complexes may pass through a series of stages. The com- 

 plexes of most recent origin contain diploid species which are common and 

 successful at least in some regions, while the polyploids may or may not 

 exceed the diploids in their range of distribution. Somewhat older polyploid 

 complexes exist in which the diploids, although containing among them all 

 the extremes of morphological variation and ecological adaptation found in 

 the complex, are nevertheless far less common than the polyploids and tend to 

 be narrow endemics. In still older complexes, nearly all the diploid species 

 have become extinct and new cycles of polyploidy have arisen, based on 

 hybridization and chromosome doubling of species which behave like diploids 

 but which have chromosome numbers of ancient polyploid derivation. Finally, 

 there exist monotypic genera containing a single species with a very high 

 chromosome number but which has no close relatives. These probably repre- 

 sent ancient polyploid complexes, of which all but one of the representative 

 species has become extinct. Examples of each of these types of polyploid 

 complexes will be discussed in turn. 



An example of a recent polyploid complex is the goat grass genus, Aegilops, 

 a group of annual grasses native to the Mediterranean region. These grasses 

 have been intensively studied because of their close relationship to wheat, at 

 least one and probably two or more species of the genus having figured in the 

 ancestry of the bread wheats. The following account is based chiefly on the 

 recent discussion of the group by Kihara (1954). 



Nine diploid species exist, which on the basis of genetic affinity can be 

 arranged into five groups. They constitute a phylogenetic series in the degree 

 of specialization of their fruiting spikelets. In the external morphology of its 

 spikelets the most primitive of these, A. mutka, is not strikingly different from 

 species of the larger and ancestral genus Agropyron. On the other hand the 

 most advanced species, A. squarrosa, A. caudata, and A. umbellulata, have 

 separately evolved complex and distinctive fruiting spikes, which constitute 

 highly efficient methods of seed dispersal. At the same time, their chromo- 

 somes have been greatly altered by means of reciprocal translocations which 

 have changed the positions of the centromeres and the relative sizes of the 

 different chromosomes of a set. These diploids have given rise by hybridiza- 



